This invention relates generally to apparatus and methods for correcting for drift errors in production of objects by stereolithography.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330 entitled "Apparatus For Production Of Three-dimensional Objects By Stereolithography," have come into use. The disclosure of U.S. Pat. No. 4,575,330 is hereby incorporated by reference, as if fully set forth herein. Basically, stereolithography is a method for automatically building complex plastic parts by successively printing cross-sections of photocurable polymer or the like on top of each other until all of the thin layers are joined together to form a whole part. With this technology, the parts are literally grown in a vat of liquid plastic. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form and for making prototypes.
Photocurable polymers ("photopolymers") change from liquid to solid in the presence of light and their photospeed with ultraviolet light (UV) is fast enough to make them practical model building materials. The material that is not polymerized when a part is made is still usable and remains in the vat as successive parts are made. An ultraviolet laser may be used which generates a small intense spot of UV light. This spot is moved across the liquid surface with a galvanometer X-Y mirror scanner. The scanner is driven by computer generated vectors or the like. Precise and complex patterns can be rapidly produced with this technique.
The laser scanner, the photopolymer vat, and an elevator, along with a controlling computer, combine together to form a stereolithography apparatus, referred to as an "SLA." An SLA is programmed to automatically make a plastic part by "drawing" one cross-section at a time, and building it up layer by layer.
Stereolithography represents an unprecedented way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross-sectional patterns, a natural data link to CAD/CAM exists.
To be effective, a stereolithography system must have information about the focus, laser beam oscillation mode, beam power, intensity distribution or profile, and scanning system drift of the drawing laser beam in order to carry out the accurate and efficient production of parts (objects made by stereolithography are known as "parts"). The beam must be in relative focus at the surface of the working photopolymer fluid. The laser mode, intensity distribution, and beam power are important to the depth and width of cure of the working fluid, as well as the scan speed. The "drift" of the scanning system must be measured and corrected periodically.
Beam profile (a profile of the intensity of the beam) measurements provide useful information about the beam because they can help accomplish the following purposes: 1. focus optics and correct astigmatism and other aberrations; 2. measure the power of the beam (needed on a day to day basis); 3. study the laser mode and changes of the mode; 4. compensate for drift of the laser scanning system; 5. allow recording of the drift for later analysis of changes; 6. automatically calibrate the scanners; 7. allow easy control of beam position for making other measurements (e.g. to independently measure the beam power in order to find the system's power calibration factor); and 8. permit the prediction of the size and shape of the cured plastic trace.
Accordingly, a need exists for an apparatus and method to correct for the drift of the mirror positioning system, particularly in connection with a stereolithographic apparatus.